1. Field of the Invention
The present invention relates to silicon nitride sintered bodies having excellent mechanical strengths at high temperatures and to a method of manufacturing the same.
2. Related Art Statement
Until recently, as for silicon nitride sintered bodies containing oxides of IVa group elements including rare earth elements as additives, for example, in Japanese Patent Publication No. 48-7486, a manufacturing method of the sintered body is disclosed which comprises preparing a mixed powder consisting essentially of at least 85 mol % of silicon nitride powdery raw material and less than 15 mol % of at least one oxide selected from the oxides of IIIa group elements, shaping the thus prepared powder, and subjecting the resulting shaped-body to a firing under an inert atmosphere. In Japanese Patent Publication No. 49-21091, the silicon nitride sintered body is also disclosed which consists of at least 50 wt % of Si.sub.3 N.sub.4, less than 50 wt % of at least one oxide selected from Y.sub.2 o.sub.3 and the oxides of La group elements, and 0.01.about.20 wt % of Al.sub.2 O.sub.3.
However, it is unlikely to obtain a sintered body having excellent high-temperature strength when adding only rare earth elements to the silicon nitride powdery raw material. On the other hand, when the sintered body contains Al.sub.2 O.sub.3 as an additive, structure of the sintered body is progressively densified, however, the softening point of its intergranular phase is lowered to considerably degrade the high-temperature strengths of the sintered body.
To obtain a sintered body having excellent high-temperature strength, the applicant of the present invention, in Japanese Patent Publication No. 63-100067, discloses a technique which provides the sintered body with excellent high-temperature strength by adding rare earth elements, having a predetermined composition and a predetermined weight ratio, to the silicon nitride powdery raw material and by specifying the crystal phase of the sintered body.
In the silicon nitride sintered body disclosed in the Japanese Patent Publication No. 63-100067, it is possible to improve the high-temperature strength of the sintered body to a certain extent, which is still lower than a room-temperature strength thereof. The result is interpreted as follows. Even performing crystallization of grain boundaries of the sintered body still leaves some amount of uncrystallized glass phase when adopting the composition disclosed in the publication. For lowering a residual amount of the uncrystallized glass phase, it is possible to propose a manufacturing method comprising a powdery raw material in which little glass phase remains in the grain boundaries by enlarging a molecule ratio of the added oxides of the rare earth elements with respect to an amount of SiO.sub.2, to which a whole amount of oxygen contained in the silicon nitride powdery raw material is converted. However, the method makes it difficult to sufficiently densify the structure of the sintered body.